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+/* Allocation from a fixed-size buffer.
+ Copyright (C) 2017-2018 Free Software Foundation, Inc.
+ This file is part of the GNU C Library.
+
+ The GNU C Library is free software; you can redistribute it and/or
+ modify it under the terms of the GNU Lesser General Public
+ License as published by the Free Software Foundation; either
+ version 2.1 of the License, or (at your option) any later version.
+
+ The GNU C Library is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ Lesser General Public License for more details.
+
+ You should have received a copy of the GNU Lesser General Public
+ License along with the GNU C Library; if not, see
+ <http://www.gnu.org/licenses/>. */
+
+/* Allocation buffers are used to carve out sub-allocations from a
+ larger allocation. Their primary application is in writing NSS
+ modules, which receive a caller-allocated buffer in which they are
+ expected to store variable-length results:
+
+ void *buffer = ...;
+ size_t buffer_size = ...;
+
+ struct alloc_buffer buf = alloc_buffer_create (buffer, buffer_size);
+ result->gr_name = alloc_buffer_copy_string (&buf, name);
+
+ // Allocate a list of group_count groups and copy strings into it.
+ char **group_list = alloc_buffer_alloc_array
+ (&buf, char *, group_count + 1);
+ if (group_list == NULL)
+ return ...; // Request a larger buffer.
+ for (int i = 0; i < group_count; ++i)
+ group_list[i] = alloc_buffer_copy_string (&buf, group_list_src[i]);
+ group_list[group_count] = NULL;
+ ...
+
+ if (alloc_buffer_has_failed (&buf))
+ return ...; // Request a larger buffer.
+ result->gr_mem = group_list;
+ ...
+
+ Note that it is not necessary to check the results of individual
+ allocation operations if the returned pointer is not dereferenced.
+ Allocation failure is sticky, so one check using
+ alloc_buffer_has_failed at the end covers all previous failures.
+
+ A different use case involves combining multiple heap allocations
+ into a single, large one. In the following example, an array of
+ doubles and an array of ints is allocated:
+
+ size_t double_array_size = ...;
+ size_t int_array_size = ...;
+
+ void *heap_ptr;
+ struct alloc_buffer buf = alloc_buffer_allocate
+ (double_array_size * sizeof (double) + int_array_size * sizeof (int),
+ &heap_ptr);
+ _Static_assert (__alignof__ (double) >= __alignof__ (int),
+ "no padding after double array");
+ double *double_array = alloc_buffer_alloc_array
+ (&buf, double, double_array_size);
+ int *int_array = alloc_buffer_alloc_array (&buf, int, int_array_size);
+ if (alloc_buffer_has_failed (&buf))
+ return ...; // Report error.
+ ...
+ free (heap_ptr);
+
+ The advantage over manual coding is that the computation of the
+ allocation size does not need an overflow check. In case of an
+ overflow, one of the subsequent allocations from the buffer will
+ fail. The initial size computation is checked for consistency at
+ run time, too. */
+
+#ifndef _ALLOC_BUFFER_H
+#define _ALLOC_BUFFER_H
+
+#include <inttypes.h>
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdlib.h>
+#include <sys/param.h>
+
+/* struct alloc_buffer objects refer to a region of bytes in memory of a
+ fixed size. The functions below can be used to allocate single
+ objects and arrays from this memory region, or write to its end.
+ On allocation failure (or if an attempt to write beyond the end of
+ the buffer with one of the copy functions), the buffer enters a
+ failed state.
+
+ struct alloc_buffer objects can be copied. The backing buffer will
+ be shared, but the current write position will be independent.
+
+ Conceptually, the memory region consists of a current write pointer
+ and a limit, beyond which the write pointer cannot move. */
+struct alloc_buffer
+{
+ /* uintptr_t is used here to simplify the alignment code, and to
+ avoid issues undefined subtractions if the buffer covers more
+ than half of the address space (which would result in differences
+ which could not be represented as a ptrdiff_t value). */
+ uintptr_t __alloc_buffer_current;
+ uintptr_t __alloc_buffer_end;
+};
+
+enum
+ {
+ /* The value for the __alloc_buffer_current member which marks the
+ buffer as invalid (together with a zero-length buffer). */
+ __ALLOC_BUFFER_INVALID_POINTER = 0,
+ };
+
+/* Internal function. Terminate the process using __libc_fatal. */
+void __libc_alloc_buffer_create_failure (void *start, size_t size);
+
+/* Create a new allocation buffer. The byte range from START to START
+ + SIZE - 1 must be valid, and the allocation buffer allocates
+ objects from that range. If START is NULL (so that SIZE must be
+ 0), the buffer is marked as failed immediately. */
+static inline struct alloc_buffer
+alloc_buffer_create (void *start, size_t size)
+{
+ uintptr_t current = (uintptr_t) start;
+ uintptr_t end = (uintptr_t) start + size;
+ if (end < current)
+ __libc_alloc_buffer_create_failure (start, size);
+ return (struct alloc_buffer) { current, end };
+}
+
+/* Internal function. See alloc_buffer_allocate below. */
+struct alloc_buffer __libc_alloc_buffer_allocate (size_t size, void **pptr)
+ __attribute__ ((nonnull (2)));
+
+/* Allocate a buffer of SIZE bytes using malloc. The returned buffer
+ is in a failed state if malloc fails. *PPTR points to the start of
+ the buffer and can be used to free it later, after the returned
+ buffer has been freed. */
+static __always_inline __attribute__ ((nonnull (2)))
+struct alloc_buffer alloc_buffer_allocate (size_t size, void **pptr)
+{
+ return __libc_alloc_buffer_allocate (size, pptr);
+}
+
+/* Mark the buffer as failed. */
+static inline void __attribute__ ((nonnull (1)))
+alloc_buffer_mark_failed (struct alloc_buffer *buf)
+{
+ buf->__alloc_buffer_current = __ALLOC_BUFFER_INVALID_POINTER;
+ buf->__alloc_buffer_end = __ALLOC_BUFFER_INVALID_POINTER;
+}
+
+/* Return the remaining number of bytes in the buffer. */
+static __always_inline __attribute__ ((nonnull (1))) size_t
+alloc_buffer_size (const struct alloc_buffer *buf)
+{
+ return buf->__alloc_buffer_end - buf->__alloc_buffer_current;
+}
+
+/* Return true if the buffer has been marked as failed. */
+static inline bool __attribute__ ((nonnull (1)))
+alloc_buffer_has_failed (const struct alloc_buffer *buf)
+{
+ return buf->__alloc_buffer_current == __ALLOC_BUFFER_INVALID_POINTER;
+}
+
+/* Add a single byte to the buffer (consuming the space for this
+ byte). Mark the buffer as failed if there is not enough room. */
+static inline void __attribute__ ((nonnull (1)))
+alloc_buffer_add_byte (struct alloc_buffer *buf, unsigned char b)
+{
+ if (__glibc_likely (buf->__alloc_buffer_current < buf->__alloc_buffer_end))
+ {
+ *(unsigned char *) buf->__alloc_buffer_current = b;
+ ++buf->__alloc_buffer_current;
+ }
+ else
+ alloc_buffer_mark_failed (buf);
+}
+
+/* Obtain a pointer to LENGTH bytes in BUF, and consume these bytes.
+ NULL is returned if there is not enough room, and the buffer is
+ marked as failed, or if the buffer has already failed.
+ (Zero-length allocations from an empty buffer which has not yet
+ failed succeed.) */
+static inline __attribute__ ((nonnull (1))) void *
+alloc_buffer_alloc_bytes (struct alloc_buffer *buf, size_t length)
+{
+ if (length <= alloc_buffer_size (buf))
+ {
+ void *result = (void *) buf->__alloc_buffer_current;
+ buf->__alloc_buffer_current += length;
+ return result;
+ }
+ else
+ {
+ alloc_buffer_mark_failed (buf);
+ return NULL;
+ }
+}
+
+/* Internal function. Statically assert that the type size is
+ constant and valid. */
+static __always_inline size_t
+__alloc_buffer_assert_size (size_t size)
+{
+ if (!__builtin_constant_p (size))
+ {
+ __errordecl (error, "type size is not constant");
+ error ();
+ }
+ else if (size == 0)
+ {
+ __errordecl (error, "type size is zero");
+ error ();
+ }
+ return size;
+}
+
+/* Internal function. Statically assert that the type alignment is
+ constant and valid. */
+static __always_inline size_t
+__alloc_buffer_assert_align (size_t align)
+{
+ if (!__builtin_constant_p (align))
+ {
+ __errordecl (error, "type alignment is not constant");
+ error ();
+ }
+ else if (align == 0)
+ {
+ __errordecl (error, "type alignment is zero");
+ error ();
+ }
+ else if (!powerof2 (align))
+ {
+ __errordecl (error, "type alignment is not a power of two");
+ error ();
+ }
+ return align;
+}
+
+/* Internal function. Obtain a pointer to an object. */
+static inline __attribute__ ((nonnull (1))) void *
+__alloc_buffer_alloc (struct alloc_buffer *buf, size_t size, size_t align)
+{
+ if (size == 1 && align == 1)
+ return alloc_buffer_alloc_bytes (buf, size);
+
+ size_t current = buf->__alloc_buffer_current;
+ size_t aligned = roundup (current, align);
+ size_t new_current = aligned + size;
+ if (aligned >= current /* No overflow in align step. */
+ && new_current >= size /* No overflow in size computation. */
+ && new_current <= buf->__alloc_buffer_end) /* Room in buffer. */
+ {
+ buf->__alloc_buffer_current = new_current;
+ return (void *) aligned;
+ }
+ else
+ {
+ alloc_buffer_mark_failed (buf);
+ return NULL;
+ }
+}
+
+/* Obtain a TYPE * pointer to an object in BUF of TYPE. Consume these
+ bytes from the buffer. Return NULL and mark the buffer as failed
+ if there is not enough room in the buffer, or if the buffer has
+ failed before. */
+#define alloc_buffer_alloc(buf, type) \
+ ((type *) __alloc_buffer_alloc \
+ (buf, __alloc_buffer_assert_size (sizeof (type)), \
+ __alloc_buffer_assert_align (__alignof__ (type))))
+
+/* Internal function. Obtain a pointer to an object which is
+ subsequently added. */
+static inline const __attribute__ ((nonnull (1))) void *
+__alloc_buffer_next (struct alloc_buffer *buf, size_t align)
+{
+ if (align == 1)
+ return (const void *) buf->__alloc_buffer_current;
+
+ size_t current = buf->__alloc_buffer_current;
+ size_t aligned = roundup (current, align);
+ if (aligned >= current /* No overflow in align step. */
+ && aligned <= buf->__alloc_buffer_end) /* Room in buffer. */
+ {
+ buf->__alloc_buffer_current = aligned;
+ return (const void *) aligned;
+ }
+ else
+ {
+ alloc_buffer_mark_failed (buf);
+ return NULL;
+ }
+}
+
+/* Like alloc_buffer_alloc, but do not advance the pointer beyond the
+ object (so a subseqent call to alloc_buffer_next or
+ alloc_buffer_alloc returns the same pointer). Note that the buffer
+ is still aligned according to the requirements of TYPE. The effect
+ of this function is similar to allocating a zero-length array from
+ the buffer. */
+#define alloc_buffer_next(buf, type) \
+ ((const type *) __alloc_buffer_next \
+ (buf, __alloc_buffer_assert_align (__alignof__ (type))))
+
+/* Internal function. Allocate an array. */
+void * __libc_alloc_buffer_alloc_array (struct alloc_buffer *buf,
+ size_t size, size_t align,
+ size_t count)
+ __attribute__ ((nonnull (1)));
+
+/* Obtain a TYPE * pointer to an array of COUNT objects in BUF of
+ TYPE. Consume these bytes from the buffer. Return NULL and mark
+ the buffer as failed if there is not enough room in the buffer,
+ or if the buffer has failed before. (Zero-length allocations from
+ an empty buffer which has not yet failed succeed.) */
+#define alloc_buffer_alloc_array(buf, type, count) \
+ ((type *) __libc_alloc_buffer_alloc_array \
+ (buf, __alloc_buffer_assert_size (sizeof (type)), \
+ __alloc_buffer_assert_align (__alignof__ (type)), \
+ count))
+
+/* Internal function. See alloc_buffer_copy_bytes below. */
+struct alloc_buffer __libc_alloc_buffer_copy_bytes (struct alloc_buffer,
+ const void *, size_t)
+ __attribute__ ((nonnull (2)));
+
+/* Copy SIZE bytes starting at SRC into the buffer. If there is not
+ enough room in the buffer, the buffer is marked as failed. No
+ alignment of the buffer is performed. */
+static inline __attribute__ ((nonnull (1, 2))) void
+alloc_buffer_copy_bytes (struct alloc_buffer *buf, const void *src, size_t size)
+{
+ *buf = __libc_alloc_buffer_copy_bytes (*buf, src, size);
+}
+
+/* Internal function. See alloc_buffer_copy_string below. */
+struct alloc_buffer __libc_alloc_buffer_copy_string (struct alloc_buffer,
+ const char *)
+ __attribute__ ((nonnull (2)));
+
+/* Copy the string at SRC into the buffer, including its null
+ terminator. If there is not enough room in the buffer, the buffer
+ is marked as failed. Return a pointer to the string. */
+static inline __attribute__ ((nonnull (1, 2))) char *
+alloc_buffer_copy_string (struct alloc_buffer *buf, const char *src)
+{
+ char *result = (char *) buf->__alloc_buffer_current;
+ *buf = __libc_alloc_buffer_copy_string (*buf, src);
+ if (alloc_buffer_has_failed (buf))
+ result = NULL;
+ return result;
+}
+
+#ifndef _ISOMAC
+libc_hidden_proto (__libc_alloc_buffer_alloc_array)
+libc_hidden_proto (__libc_alloc_buffer_allocate)
+libc_hidden_proto (__libc_alloc_buffer_copy_bytes)
+libc_hidden_proto (__libc_alloc_buffer_copy_string)
+libc_hidden_proto (__libc_alloc_buffer_create_failure)
+#endif
+
+#endif /* _ALLOC_BUFFER_H */